1. Field of the Invention
The present invention relates to a coaxial resonator. More specifically, the present invention relates to a 1/4 wave length coaxial TEM resonator comprising a dielectric unit between an outer conductor and an inner conductor.
2. Description of the Prior Art
FIG. 1 is a sectional view of a 1/4 wave length coaxial TEM resonator of the prior art and FIG. 2 is a sectional view taken along the line II--II in FIG. 1. Referring to FIGS. 1 and 2, a conventional 1/4 wave length coaxial TEM resonator will be described. A dielectric unit 10 is made of a ceramic material of a titanium oxide group and formed in a hollow cylindrical shape having an outer wall surface and an inner wall surface. The dielectric unit 10 is provided on the outer wall surface with a cylindrical metallic member of an electrical conductor to constitute an outer conductor 21. More specifically, the outer conductor 21 is fitted onto the dielectric unit 10 so that the inner wall surface of the outer conductor 21 closely fits to the outer wall surface of the dielectric unit 10. The axial length of the outer conductor 21 is selected to be a 1/4 wave length of a propagating electric wave. The axial length of the dielectric unit 10 is selected to be approximately two-thirds of the total axial length of the outer conductor 21. Accordingly, a cavity 22 of approximately one third of the axial length of the outer conductor 21 is formed in the portion close to one end of the outer conductor 21, where no dielectric member exists.
A central rod 23 made of ceramic is inserted into the hollow portion of the dielectric unit 10 for the purpose of reinforcement. The central rod 23 is selected to be of the same axial length as that of the outer conductor 21. A silver paste having a good high frequency characteristic and of a good adhesiveness to the dielectric unit 10 is formed, by baking, on the outer wall surface of the central rod 23, thereby forming an inner conductor 24. The 1/4 wave length coaxial resonator thus formed is housed in a metallic casing formed with a bottomed hole having an inner diameter which is equal to the outer diameter of the outer conductor 21. One end of the outer conductor 21 and one end of the inner conductor 24 are short-circuited by the casing, whereby a short-circuit end is formed.
In such conventional 1/4 wave length coaxial TEM resonators the effective dielectric constant of the resonator is considerably decreased at the cavity portion 22 formed at the short-circuit end 25. Usually the electric field of the fundamental wave is zero or nearly zero at the short-circuit end of the coaxial resonator. Therefore, even if the dielectric constant of the substance such as air or vacuum existing between the outer conductor 21 and the inner conductor 24 is low, little influence is exerted upon the resonant frequency. However, the electric field of the third harmonic is large at the short-circuit end 25 and the third harmonic is influenced by a small effective dielectric constant of the substance or vacuum existing at the cavity portion 22. Therefore, resonance of the third harmonic which is a cause to degrade a spurious characteristic becomes liable to occur at a higher frequency region and a spurious characteristic can be improved as compared with a coaxial resonator which does not comprise the cavity portion 22.
Meanwhile, in the case of the FIG. 1 coaxial resonator, the inner conductor 24 has been formed by baking silver. However, usually a dopant material is employed in baking silver and the fact that pure silver is not employed degrades electrical conductivity and increases loss and thus reduces the quality factor.
In some cases, two 1/4 wave length coaxial resonators are combined so that the short-circuit ends thereof may be electrically coupled to serve as a half-wave coaxial resonator. In such cases, it is difficult to electrically couple the short-circuit ends of the above described 1/4 wave length coaxial resonators, with the result that the electrical conductivity at the contacting portion is degraded.
Furthermore, in order to implement a 1/2 wave length coaxial resonator using two 1/4 wave length coaxial resonators, it is necessary to inductively couple these 1/4 wave length coaxial resonators at the respective short-circuit ends, with the result that some inductive coupling means need be provided for the purpose of inductively coupling the two coaxial resonators. Therefore, a problem is encountered that a structure for implementing a 1/2 wave length coaxial resonator using two conventional 1/4 wave length coaxial resonators becomes complicated and expensive.